When posting artwork, I generally try to keep my files under 200 KB...JPEG format still allows for a lot of detail at that size. Ideally, if you can keep images to around/under 150 KB, that keeps the download time manageable for everyone (especially considering people with 56 Kb or slower modems).
Another generally good guideline would be to try to keep the total image size (in pixels) no larger than about 1024X768...that way, when someone fullviews the image, they aren't forced to scroll all over the place.
Did this help? I don't really think there's a true protocol involved in posting to DA...just be considerate of other peoples' technological limitations...primarily bandwidth, as that seems to be one of the biggest issues for most of us.